For measuring pressure in a tire of an automobile, pressure sensors are used, which are installed in car tires. When an automobile is moving, centrifugal accelerations of up to 2,000 g occur in car tires. These accelerations can lead to a significant measurement error in tire pressure sensors, which are also moved in the respective tire.
Normally, the pressure sensor consists of a flexible membrane, which bends under the influence of an external pressure. This bending (deformation) is measured electronically and is a measure for the external pressure.
Due to the own weight of a membrane, an acceleration acting vertically to the membrane also leads to a bending. Particularly in surface mechanical silicon pressure sensors, which require a protective layer, such as a gel, for media separation, the effect accompanying the additional membrane bending is increased by the protective layer (media protection) deposited on the membrane, such that a significant measurement error occurs when using such sensors. Due to the acceleration acting vertically to the membrane, however, there is also a measurement error in sensors that require no protective layer. For example with sensors, which are setup in bulk micromechanic, the pressure can be brought up from the backside of the substrate, so that in sensors set-up that way mainly the own weight of the membrane is responsible for the measurement errors resulting from the acceleration.
Above that, the acceleration acting transversally to the membrane and the accompanying bending of the membrane is always dependent on a position of the tire pressure sensor in a vehicle. If the tire pressure sensor is less distant from an axis of a wheel, the centrifugal acceleration acting on the membrane is lower than in the case of a tire pressure sensor, which is further away from the axis of a wheel, since the centrifugal acceleration depends linearly on an installation radius. If, for example, tire pressure sensors are installed later, different measurements will occur if an exactly equal installation height in all tire pressure sensors is not kept, so that, for example, four different measurement values would be indicated with a tire pressure, which is equal in all four wheels, since the tire pressure sensors each respectively experience a different centrifugal acceleration. The same effect occurs also when the installation height of the respective tire pressure sensor is kept exactly and when an angle between normal on the sensor membrane and the respective wheel radius is different for each one of the tire pressure sensors. In that case, a respectively different force, which also leads to a measurement error, acts vertically on the respective membrane during acceleration.
For avoiding the above-described problem, the tire pressure sensors can already be calibrated exactly during installation, so that, for example, always the same force acts on all four sensors during acceleration. This approach, however, is expensive and difficult to be realized with a later installation of the tire sensors.
Above that, a settlement of a wheel speed in a central unit can be performed for compensating the acceleration errors. Therefore, apart from a pressure measuring value p, a wire speed ω and a sensor specific constant γ should be known to the central unit, which indicates a corrected pressure value, and which establishes a connection between acceleration and pressure error. For a sensor setup in surface micromechanic with a gel thickness h1 over a sensor cell and a gel density ρ1, the following applies to the pressure measurement value p:p=p0+ah1ρ1,wherein a is an acceleration normal to the membrane level and p0 a real internal pressure of a tire.
The factor γ is therefore calculated to γ=h1 ρ1 cos α, wherein α is an angle between a normal on a sensor membrane and a radius of a wheel. A centrifugal acceleration at a wheel speed ω can be determined based on the connection a=r ω2, wherein a is an installation radius.
In the above-described method, therefore, the central unit has to exactly know an installation position of the sensors. Above that, the approach is only suitable for later installed systems (after marked), when the systems can be provided with a value of the wheel speed.